The rising cost of imported petroleum fuels has intensified the search for more efficient and convenient systems for obtaining heat from wood. The more traditional approach to burning wood fuel as logs or sticks whether in space heating wood stoves or more sophisticated central heating wood fuel furnaces suffer the disadvantage that they are non-automatic and require periodic hand loading of the log wood fuel. While large scale automated log fuel burners have been in existance for many years, there are at present no suitable systems available for residential and small scale use with heat outputs in the range, for example of 50,000 to 100,000 BTU/hr.
Wood chips offer an alternative form of wood fuel more suitable for automatic feed furnaces than the more traditional log wood fuel. This fragmented form of wood fuel may be derived from a number of sources including whole trees, brush, industrial waste wood, and the tops and branches left in the woods after logging operations. Under present harvesting methods it is estimated that one third of the weight of harvested trees is left in the forest, mainly as cull logs, tops and branches. If all these logging residues could be efficiently used, they would supply a substantial portion of residential heating needs in regions with abundant timber resources. The advantage of using this resource in fragmented or chipped form is that it allows reduction of all types of wood to a useable form.
By chipping or pellitizing the wood resource into a fragmented form of substantially uniform range of fragment size, continuous feed of wood fuel at a rate commensurate with residential and small scale heating applications is possible. The physical characteristics of wood chips and pellitized wood material combined with the slow steady feed requirements and scaling factors demand a furnace system of unique design. Furthermore because of the nearly ubiquitous distribution of conventional oil furnaces throughout the northern climates it would be advantageous to incorporate or retrofit such oil furnaces into the wood type fuel fired central heating furnace system.